Electrical circuits need to be protected in a fault situation. Typical fault situations include overload or short circuit resulting in overcurrents or excess currents, i.e. when a larger than intended electric current flows in a circuit. In an overcurrent situation, excessive heat is generated in a conductor, which might result in circuit damage or even fire. In order to protect electrical circuits in a fault situation described above, circuit breakers can be utilized. A basic function of a circuit breaker is to interrupt electrical current by breaking or disconnecting the circuit immediately after a fault condition has been detected.
Circuit breakers can be provided as mechanical switches. These switches typically have at least two contact members which are initially pressed against each other and conduct the current in normal operation. In case of a fault, due to the overcurrent, the contact members and/or insulating gas surrounding the contact members is or are heated up, until the material of the contact members and/or of the surrounding gas is ionized and becomes conductive, i.e. reaches a plasma state. When a mechanism which separates the two contact members of the switch is triggered, the separation of the contact members does not interrupt the flow of current immediately, since the current continues to flow through a gap within the plasma medium. Thereby, an electric arc is created. The arc can only be sustained, if the current, and with it the electric heating of the plasma, is sufficiently high. This is typically the case for fault current conditions.
In order to interrupt the flow of current, the arc must be extinguished. This can be achieved by decreasing the current and with it the heating power below a certain threshold, below which the heating is not sufficient to sustain the arc. The plasma cools down and loses its conductivity. Such a situation can typically only be reached around a current zero crossing of the current, as with vanishing current the heating of the plasma disappears, as well. Hence, conventional AC circuit breakers are switching off the current at a zero crossing.
In general, interrupting DC currents (direct currents) is difficult compared to interrupting AC currents (alternating currents). This is caused by the lack of natural current zero crossings in a DC case as opposed to an AC case. Therefore, a DC circuit breaker has to create first a zero crossing and then to interrupt at current zero. The standard solution today for voltages up to about 1.5 kV makes use of an arc chute building up typically twice the grid voltage. This counter voltage to the grid voltage drives the current towards zero, where the arc can be extinguished. However, the minimal arc chute voltage needed is larger than the grid voltage. Hence, this concept is limited to low voltages, because otherwise the number of splitter plates needed for building up enough voltage becomes too large.
As an alternative, a concept based on a passive resonance circuit can be used. FIG. 1 depicts a DC circuit breaker 110, wherein an LC-circuit branch 130, comprising an inductor 132 and a capacitor 134, is connected in parallel to arc contact members 120a, 120b. 112 represents a grid resistance Rg and 114 a grid impedance Lg. 116 is a residual breaker.
FIG. 2 shows the currents as a function of time, wherein Ig is the grid current, e.g. the current flowing through Lg, Ir is the resonance branch current, i.e. the current flowing in the resonance branch 130, and Ia is the arc current, i.e. the current flowing in the arc 122, wherein Ia=Ig−Ir.
Before a fault (at times t<tf, with tf=fault time), all current flows through the closed contact members 120a, 120b. After the fault (tf<t<tcs, with tcs=time of contact separation), the current starts to rise linearly (limited by the grid impedance Lg). As soon as the arc contact members 120a, 120b separate (at contact separation time tcs), an arc 122 is formed.
After the contact separation (t>tcs), due to an arc voltage, some of the current is diverted into the resonance branch 130, where the resonance branch current Ir starts to flow, to increase and to charge the capacitor 134. Once the capacitor 134 is charged to the voltage of the arc, the resonance branch current Ir starts to decrease. Ir cannot disappear immediately due to the inductance L of the inductor 132. Once the resonance branch current Ir reaches zero, the capacitor 134 is charged to about twice the arc voltage. Hence, the resonance branch current Ir starts to flow into the opposite direction and to increase in absolute value. The arc current Ia then increases, as well.
Due to the negative arc characteristics (i.e. low arc current leads to high arc voltage and vice versa high arc current leads to low arc voltage), the arc voltage is lower at high values of the arc current Ia. Hence, the capacitor 134 discharges to a low voltage. The resonance branch current Ir reaches its minimum when the capacitor 134 is discharged. Afterwards, the resonance branch current Ir rises again, while the capacitor 134 is discharged further to negative voltages.
This cycle of charging the capacitor 134 with higher arc voltage then discharging it with lower arc voltage, continues and causes oscillations of the resonance branch current Ir, the arc current Ia and the capacitor voltage with ever increasing amplitudes.
As soon as the arc current Ia passes zero (t=tz with tz=time of current zero crossing), the arc 122 can be extinguished.
The object of the present invention is to provide an improved DC circuit breaker for more effective, fast and reliable interrupting of currents.